A staggering one million burn injuries occur in the United States every year and over 45,000 require hospitalization. Severe extensive burns covering over 60% of the total body surface area account for 4% of the admissions, about 2,000 patients per year. Burn injuries are especially common among children, with nearly 80,000 infants being treated annually, making burns the third leading cause of childhood injury-related mortality in both Canada and the United States (Hammig and Ogletree (2006) Am J Health Behav 30:259-267). Second and third degree burns cause severe damage to the skin dermal layer.
Epidermal regeneration is a complex process in which keratinocytes proliferate and migrate to cover up the defect in the epidermis restoring lost barrier function of the skin. The molecular mechanisms involved in this process are still not completely understood but peptide growth factors, e.g., keratinocyte growth factor (KGF), have been shown to play an important role in epidermal regeneration (Hardwicke et al. (2008) Surgeon 6(3):172-177; McDowall et al. (2008) Cytokine & Growth Factor Rev 19(5-6):415-426; Koria and Andreadis (2007) Am J Phys 293(3):C1020-1031; Werner and Grose (2003) Physiological Rev 83(3):835-870; Grazul-Bilska et al. (2003) Drugs Today (Bare) 39(10):787-800; Curtsinger et al. (1989) Surgery, Gynecology & Obstetrics 168(6):517-522; Brown et al. (1986) J Exp Med 163(5):1319-1324).
KGF is a monomeric peptide belonging to the fibroblast growth factor family (FGF-7) and plays a prominent role in epidermal morphogenesis and wound healing (Werner et al. (1994) Science 266(5186):819-822; Beer et al. (2000) J Investigative Dermatol Symp Proc 5(1):34-39). It is mainly expressed by cells of mesenchymal origin such as fibroblasts, micro-vascular endothelial and smooth muscle cells but affects epithelial cells (Winkles et al. (1997) J Cell Physiol 173(3):380-386; Smola et al. (1993) J Cell Biol 122(2):417-429). This paracrine mode of action of KGF on epithelial cells is mediated through the KGF receptor (KGFR or FGFRIIIb), a splice variant of the FGF-2 receptor encoded by the gene fgfr-2 (Mild et al. (1991) Science 251(4989):72-75; Miki et al. (1992) Proc Natl Acad Sci USA 89(1):246-250). KGF is present at very low levels in skin under normal conditions but it is highly up-regulated after injury (Werner et al. (1992) Proc Natl Acad Sci USA 89(15):6896-6900). While wound healing of KGFR deficient mice was severely impaired (Werner et al. (1994) Science 266(5186):819-822), mice lacking KGF healed at normal rates (Guo et al. (1996) Genes Dev 10(2):165-175), possibly due to the compensatory action by other members of the FGF family e.g. FGF-10 (Beer et al. (1997) Oncogene 15(18):2211-2218) or FGF-22 (Beyer et al. (2003) Exp Cell Res 287(2):228-236). Despite such redundancies exogenous KGF significantly enhanced re-epithelialization of full and partial thickness wounds in porcine and rabbit ear wound models (Staiano-Coico et al. (1993) J Exp Med 178(3):865-878; Pierce et al. (1994) J Exp Med 179(3):831-840). In addition to re-epithelialization, exogenous delivery of KGF enhanced granulation tissue formation in an ischemic rabbit ear wound model (Gillis et al. (1999) J Cell Sci 112(Pt 12):2049-2057) and injection of KGF DNA accelerated wound closure and reduced inflammation in a diabetic mouse model (Marti et al. (2004) Gene Ther 2004 11(24):1780-1785). Furthermore, development of engineered skin equivalents with KGF-expressing human keratinocytes showed changes in epidermal structure and morphology including hyper-thickening (Andreadis et al. (2001) FASEB J 15(6):898-906).
Initial clinical studies on topical application of growth factor preparations to accelerate epidermal regeneration gave controversial results (Brown et al. (1991) Plastic Reconstructive Surgery 88(2):189-194; discussion 195-186; Brown et al. (1988) Annals Surgery 208(6):788-794; Brown et al. (1989) New England J Med 321(2):76-79). This was largely due to the limited bioavailability of topically delivered growth factors in the wound environment (Curtsinger et al. (1989) Surgery, Gynecology & Obstetrics 168(6):517-522; Brown et al. (1989) New England J Med 321(2):76-79). Such topical treatment does not keep the growth factor localized in the wound and necessitates the use of large amounts of growth factor (Marti et al. (2008) Meth Mol Biol 423:383-391). This is not only associated with high cost but also potential side effects such as vascularization of non target tissues or tumors (Epstein et al. (2001) Cardiovascular Res 49(3):532-542). Therefore, there is a need to design growth factor delivery systems that ensure presence of the growth factor during the epidermal regeneration process. Several groups have devised strategies to address this issue. Current strategies include chemical conjugation of growth factors to extra-cellular molecules like fibrin or collagen (Geer et al. (2005) Am J Pathology 167(6):1575-1586; Curtsinger et al. (1989) Surgery, Gynecology & Obstetrics 168(6):517-522), delivery vehicles like multi-lamellar vesicles (Brown et al. (1988) Annals Surgery 208(6):788-794) or gene therapy which involves delivery of a growth factor encoding plasmid to the target cells (Davidson (2008) J Invest Derm 128(6):1360-1362; Escamez et al. (2008) J Invest Derm 128(6):1565-1575; Hirsch et al. (2007) Front Biosci 12:2507-2518), or transplanting genetically modified cells that over-express KGF (Kopp et al. (2004) Mol Ther 10(1):86-96). Chemical conjugation of growth factors requires large quantities of the growth factor, increasing the cost of treatment. While gene therapy has proven effective in some cases, several extra cellular and intracellular barriers must be overcome in order to obtain successful expression of the transgene (Hirsch et al. (2007) Front Biosci 12:2507-2518). Moreover, once these barriers have been successfully overcome, there is generally little control over the duration of expression or amount of growth factor produced by the transfected cell (Yao and Eriksson (2000) Wound Repair Regen 8(6):443-451; Byrnes et al. (1997) J Phys Chem B 101(51):11007-11028). Furthermore, in instances where the delivery of multiple growth factors is desired, the recalcitrance of some transfected cells to further transfections may be an issue. Therefore, there is a great demand for devising strategies that improve and accelerate the process of wound healing.